Surface Treating Method for Cutting Tools

ABSTRACT

In a cutting tool, if the outermost ceramic coating layer is a κ-Al 2 O 3  coating layer, then certain microns of the κ-Al 2 O 3  layer will be transformed into an α-Al 2 O 3  by instantaneous melting, vaporization and solidification. Further, if the outermost coating layer of the ceramic coating layers is an α-Al 2 O 3  coating layer, then the surface roughness will be enhanced since at least a portion of it will be melted, wherein the melted surface will be solidified with its surface flattened by the surface tension provided in a melted state.

TECHNICAL FIELD

The present invention generally relates to a method of treating asurface of a cutting tool, and more particularly to a surface treatmentmethod for a ceramic coating layer of a coated cutting tool through anelectron beam.

BACKGROUND ART

Ceramic is widely used as a coating material for hard metal cuttingtools since its mechanical properties do not easily deteriorate even athigh temperatures. Although there are various ceramic coating materials,Alumina (Al₂O₃) is particularly well known for its excellent thermalstability, hardness and abrasion resistance among such materials.

Out of various phases of Alumina, α-Al₂O₃ has excellent thermalresistance and abrasion resistance. Thus, it is preferable to form anα-Al₂O₃ coating layer on a surface of a cutting tool. However, comparedto the formation of a κ-Al₂O₃ coating layer, it is more difficult toform an α-Al₂O₃ coating layer on a surface of a cutting tool. Further,an α-Al₂O₃ coating layer is allowed to form only after a certain coatingmaterial such as TiCNO is formed on the substrate of a cutting tool.Transforming the phase of the κ-Al₂O₃ coating layer, which is molded onthe surface of a cutting tool, into an α phase was considered. However,it is difficult to realize this since such a transformation requiresinitially forming a κ-Al₂O₃ coating layer on the surface of a cuttingtool, melting the cutting tool at a temperature of more than 2000° C.and then instantaneously cooling the cutting tool. Also, the cuttingtool melted at a temperature of more than 2000 degrees Celsius can besuddenly cooled. Thus, the K phase has been traditionally used as analumina coating layer.

A cutting tool with bad surface roughness tends to have a high cuttingresistance and easily adheres to a workpiece. Thus, the service life isshortened. A ceramic coating such as α-Al₂O₃ has bad surface roughnesscompared to other coating materials for a cutting tool. Thus, such aceramic coating needs to be surface-treated to obtain an improvedsurface roughness. Various methods such as blasting, polishing,brushing, etc., are known as a surface treating method for improvingsurface roughness.

However, a ceramic coating material such as α-Al₂O₃ is very difficult tosurface-treat in the above-mentioned methods due to its hardness. Thus,it takes much time and effort to perform such surface treatingoperations. Furthermore, it has limits on the improvement of surfaceroughness.

DISCLOSURE Technical Problem

The present invention is conceived in order to solve the above problems.It is an object of the present invention to provide a simple andreliable method of forming an α-Al₂O₃ coating layer on a surface of acutting tool.

It is another object of the present invention to provide a simple andreliable method for dramatically enhancing the surface roughness of anα-Al₂O₃ ceramic coating layer formed on a cutting tool.

Furthermore, the present invention seeks to provide a cutting tool withan improved α-Al₂O₃ coating layer on its surface.

Technical Solution

In order to achieve the above objects and other objects, a surfacetreating method according to the present invention comprises the stepsof: providing one or more ceramic coating layers on a surface of acutting tool; and instantaneously melting at least a portion of theoutermost coating layer of the ceramic coating layers by irradiating anelectron beam to the ceramic coating layers.

The outermost coating layer of the one or more ceramic coating layersmay be a κ-Al₂O₃ coating layer or a TiN coating layer disposed on aκ-Al₂O₃ coating layer. A portion of the κ-Al₂O₃ coating layer isinstantaneously melted by the irradiation of an electron beam and thentransformed into α-Al₂O₃ while it is solidified. Furthermore, the TiNcoating layer is evaporated by the irradiation of the electron beam. Atleast a portion of the κ-Al₂O₃ coating layer, which was below the TiNcoating layer, is instantaneously melted and transformed into α-Al₂O₃.

Further, the outermost coating layer of one or more ceramic coatinglayers may be an α-Al₂O₃ coating layer or a TiN coating layer disposedon an α-Al₂O₃ coating layer. The α-Al₂O₃ ceramic coating layer issolidified with its surface flattened by surface tension caused while atleast a portion of the α-Al₂O₃ ceramic coating layer has been melted bythe irradiation of the electron beam. As a result, its surface roughnessmay be enhanced predominantly. The TiN coating layer is evaporated bythe irradiation of the electron beam and at least a portion of theoα-Al₂O₃ coating layer, which was below the TiN coating layer, ismelted. As such, the α-Al₂O₃ ceramic coating layer is solidified withits surface flattened by the surface tension. Thus, an excellentimprovement in surface roughness can be expected.

DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view of a conventional milling cutter with acutting insert.

FIG. 2 is a diagram of a surface-treating process in accordance with oneembodiment of the present invention.

FIG. 3 is a cross-sectional view showing ceramic coating layers of acutting insert after surface-treating through an electron beam.

FIG. 4 is an enlarged photograph of a cutting insert coated with aκ-Al₂O₃ coating layer.

FIG. 5 is an enlarged photograph of the cutting insert of FIG. 3, whichis taken after surface-treating through an electron beam.

BEST MODE FOR CARRYING OUT THE INVENTION

A surface treating method using electron beam irradiation according tothe present invention will now be described with reference to theaccompanying drawings.

The present inventors have studied technologies, which can effectivelysurface-treat an alumina coating layer of a cutting tool. Further, theyhave studied technologies, which can transform a κ-Al₂O₃ coating layerinto an oα-Al₂O₃ coating layer in a simple and stable manner, withoutdamaging the substrate of a cutting tool and other coating layersthereon.

Attempts were made to increase the stiffness of a metallic cutting toolor a surface of a metallic coating layer by irradiating thermal energysuch as a laser on the surfaces. This is to diffuse certain elementscomposing such a metallic material to the surface. However, the attemptfor thermally controlling a surface roughness of a ceramic coatinglayer, which is thermally very stable, has not yet been made. Thepresent inventors have studied regarding thermally controlling a surfacestate of the ceramic coating layer.

For example, the present inventors have studied regarding controlling asurface roughness of the ceramic coating layer of a cutting tool using alaser beam. However, they found this method to be extremely improper.That is, when CO or YAG laser beam, which has strong energy and which isdifficult to control, is irradiated to a surface of a coated cuttinginsert, both ceramic coating layer and substrate are affected by thelaser beam melting and deforming the cutting tool. In addition, sincethe laser beam having a small irradiation area irradiates its energy onrelatively broader surface of the cutting tool, scanning the surface ofthe cutting tool takes too much time. Further, such surface treatmentfails to uniformly treat the surface of an object being treated, thusleaving windings on the surface. Consequently, such surface treatment isnot suitable for use with a cutting insert.

On the contrary, unlike the laser beam, an electron beam is capable ofirradiating large areas since the irradiation area of the electron beamis not limited to small irradiation areas. Accordingly, the electronbeam may reduce a time required for surface treatment and can provideuniform surface treatment over entire surface of the object beingtreated. Further, the electron beam can be controlled such that only aportion of an outermost coating layer of a cutting tool is transformedwithout damaging substrate or any other coating layers of a cuttingtool.

FIG. 1 is a perspective view of a milling cutter with a cutting insert.A ceramic coating layer is formed on the surface of the cutting insert.The ceramic coating layer is coated on the surface of the insert with acertain thickness using known methods such as CVD, PVD and the like.

FIG. 2 illustrates an example of a device for carrying out an electronbeam surface treating process according to the present invention. Anelectron beam generating device is used in the electron beam surfacetreatment and comprises a condenser for an electron beam source, acontrolling portion, an electron beam gun, etc. In the electron beamsurface treatment, a ceramic-coated cutting insert is first placedinside the electron beam generating device. Then, the interior of theelectron beam generating device is placed under a vacuum atmosphere andan appropriate amount of Argon gas or Nitrogen gas is injected thereinto maintain the interior at 0.05 Pa vacuum level. Thereafter, anelectron beam having a diameter of 50 mm to 100 mm is generated in aCathode voltage range of 25-34KV. The electron beam shot from theelectron beam gun is accelerated in the form of a thermal electron,which is discharged for a short time (theoretically about 1 over 5hundred thousand seconds) on the ceramic coating layer of the cuttingtool. Depending on the surface reaction state of the ceramic coatinglayer, the electron beam may be irradiated repeatedly for about 1-10times.

The thermal electron, which collided onto the surface of the cuttingtool, generates high heat instantaneously since the kinetic energy ofthe accelerated electron is transformed into a thermal energy. Thetemperature of the outermost coating layer of the ceramic coating layersat this moment can rise up to about 4000° C. The outermost ceramiccoating layer is instantaneously melted from the surface to at least acertain depth (1˜5 μm). If the outermost ceramic coating layer is aκ-Al₂O₃ coating layer, then a certain amount depth (μm) of the coatinglayer is transformed into an α-Al₂O₃ layer through instant melting andsolidification of the κ-Al₂O₃. Furthermore, if the outermost coatinglayer is a TiN coating layer deposited on the κ-Al₂O₃, then the TiNcoating layer is evaporated by the electron beam irradiation. Also, aportion of the adjacent κ-Al₂O₃ coating layer is transformed intoα-Al₂O₃ by instantaneous melting and solidification. Preferably, the oneor more ceramic coating layers may be multiple layers of Ti-carbonitrideformed on the substrate of the cutting tool and a κ-Al₂O₃ coating layerformed on the multiple layers of Ti-carbonitride. Alternatively, theymay be multiple layers of Ti-carbonitride formed on the substrate of thecutting tool and a κ-Al₂O₃/TiN coating layer formed on the multiplelayers of Ti-carbonitride. Moreover, it is preferred that the one ormore ceramic coating layers have a laminated order ofTiN/MTCN/TiCN/κ-Al₂O₃ or TiN/MTCN/TiCN/κ-Al₂O₃/TiN from the substrate ofthe cutting tool. The κ-Al₂O₃ coating layer has a thickness of about 2μm to 12 μm and a thickness of at least 1 μm to 4 μm of κ-Al₂O₃ istransformed into an α-Al₂O₃ coating layer through the electron beamirradiation. Further, if the outermost coating layer of the ceramiccoating layers is a α-Al₂O₃ coating layer, then the surface roughnesswill be enhanced since its surface is solidified and then flattened bysurface tension caused while a certain amount of thickness (μm) of theα-Al₂O₃ ceramic coating layer is melted. Alternatively, if the outermostcoating layer is a TiN coating layer deposited on the α-Al₂O₃, then thesurface roughness is enhanced since the outermost TiN coating layer isevaporated by the electron beam irradiation and at least a portion ofthe α-Al₂O₃ coating layer therebelow is solidified and flattened bysurface tension caused while a certain amount of thickness (μm) of theα-Al₂O₃ ceramic coating layer is melted instantaneously after theoutermost TiN coating layer is vaporized. Preferably, the one or moreceramic coating layers may be multiple layers of Ti-carbonitride formedon the substrate of the cutting tool and a TiCNO/α-Al₂O₃ coating layerformed on the multiple layers of Ti-carbonitride. Alternatively, theymay be multiple layers of Ti-carbonitride formed on the substrate of thecutting tool and a TICNO/α-Al₂O₃/TiN coating layer formed on themultiple layers of Ti-carbonitride. Furthermore, the one or more ceramiccoating layer may be multiple layers of Ti-carbonitride formed on thesubstrate of the cutting tool and a TiCO/α-Al₂O₃ coating layer formed onthe multiple layers of Ti-carbonitride. Alternatively, they may bemultiple layers of Ti-carbonitride formed on the substrate of thecutting tool and a TICO/α-Al₂O₃/TiN coating layer formed on the multiplelayers of Ti-carbonitride. Moreover, it is preferred that the one ormore ceramic coating layers have a laminated order of the one or moreceramic coating layers may be TiN/MTCN/TiCN/TiCNO/α-Al₂O₃ orTiN/MTCN/TiCN/TiCNO/Al₂O₃/TiN from the substrate of the cutting tool.Also, it is preferred that the one or more ceramic coating layers has alaminated order of the one or more ceramic coating layers may beTiN/MTCN/TiCN/TiCO/α-Al₂O₃ or TiN/MTCN/TiCN/TiCO/α-Al₂O₃/TiN from thesubstrate of the cutting tool.

FIG. 3 is a cross-sectional view showing ceramic coating layers of acutting insert. In accordance with present invention, an electron beamis irradiated to ceramic coating layers (1 to 4) and a portion of anoutermost κ-Al₂O₃ coating layer (2) is transformed into an α-Al₂O₃ layer(1). Alternatively, if the outermost coating layer is TiN, then the TiNlayer will be evaporated by heat and at least an outer portion κ-Al₂O₃coating layer will be transformed into an α-Al₂O₃ layer 1. Since theelectron beam is irradiated for an extremely short time and reacts onlya certain depth (μm) of the coating layer surface, the other coatinglayers (3 and 4) below the κ-Al₂O₃ coating layer or the cutting toolbody (5) are not influenced by the electron beam.

Moreover, the surface roughness of the coating layer of the cutting toolis predominantly enhanced to under Ra 0.15 μm since the melted ceramiccoating layer is re-solidified while the surface is flat due to surfacetension. FIGS. 4 and 5 are photographs of the cutting insert of FIG. 3magnified 500 times by the Scanning Electron Microscope (SEM), which istaken before and after the surface-treatment through an electron beam.FIG. 4 is a photograph showing a portion of a top rake surface (6), acutting edge portion (7) and a flank face (8) of a κ-Al₂O₃ coatedcutting insert before an electron beam is discharged. FIG. 5 shows thatthe κ-Al₂O₃ coating layers formed on the top rake surface (6) and thecutting edge portion (7) is transformed into α-Al₂O₃ coating layers (6′and 7′) after the electron beam irradiation. Referring to FIGS. 4 and 5,it is confirmed that the surface roughness of the portions (6′ and 7′),which are transformed into α-Al₂O₃, is predominantly enhanced comparedto that of the portions (6 and 7) which have not been transformed.

Hereinafter, the test examples of the cutting tools, which were surfacetreated with an electron beam in accordance with the present invention,will be described.

[TEST 1]

The present inventor made a cutting performance test of cutting tools,which were treated with an electron beam in accordance of the presentinvention, as described below.

The tool life of each cutting tool was measured, wherein the tool lifeis the time spent for a wear amount of a cutting flank face of thecutting tool to reach 0.25 mm. If a breakage or chipping of the cuttingblade has occurred during cutting, then the tool life is determined asthe time spent until the breakage or chipping.

Cutting inserts A and B are used in the cutting performance tests,wherein the cutting insert A is a CNMG120412 turning insert according tothe ISO standard and is CVD coated with TiN/MTCN/TiCN/κ-Al₂O₃ coatinglayer, and wherein the cutting insert B is the same turning insertaccording to the ISO standard as the cutting insert A and is CVD coatedwith TiN/MTCN/TiCN/κ-Al₂O₃/TiN coating layer. Tests were performed oncutting inserts A and B with their surface treated by the electron beamand without surface treatments by the electron beam, respectively.

The turning inserts surface treated with the electron beam showed atransformation wherein about 1.5 μm of the κ-Al₂O₃ coating layer wastransformed into an α phase. Furthermore, when the outermost coatinglayer was TiN, the TiN coating layer was evaporated and about 1.5 μm ofthe κ-Al₂O₃ coating layer therebelow was transformed into an α phase.

The cutting conditions were as follows: cutting speed (v)=400 rpm; feedrate (f)=0.25 mm/rev; and depth of cut (d)=2.0 mm. Furthermore, eachcutting insert was tested to cut a gray cast iron having a size90×90×200 mm. Test results are shown in [Table, 1] below.

TABLE 1 coating layer cutting result CVD coated before electron beamelectron beam tool-life cutting insert surface treatment surfacetreatment dry/wet (minute) remark cutting TiN/MTCN/TiCN/ with dry 18 —insert A κ-Al₂O₃ without 12 breakage cutting TiN/MTCN/TiCN/ with wet 62— insert B κ-Al₂O₃/TiN without 48 chipping insert: CNMG120412 workmaterial: gray cast iron(HB190) cutting condition: v = 400 rpm, f = 0.25mm/rev, d = 2.0

As can be seen from [Table 1], the tool lives of the cutting inserts Aand B, the surfaces of which were not treated by the electron beam,ended by breakage or chipping before the wear amount of the cuttingflank face reached 0.25 mm. On the other hand, the cutting inserts A andB, the surfaces of which were treated by the electron beam, did not showany breakage or chipping before their wear amount reached 0.25 mm andtheir tool lives lasted even after breakage or chipping started to form.

[TEST 2]

Cutting inserts A and B were used for the test, wherein the cuttinginsert A is a CNMG120412 turning insert according to the ISO standardand is CVD coated with TiN/MTCN/TiCN/κ-Al₂O₃ coating layer, and whereinthe cutting insert B is the same turning insert as cutting insert Aaccording to the ISO standard and is CVD coated withTiN/MTCN/TiCN/κ-Al₂O₃/TiN coating layer. The workpiece of the test wascarbon steel and the other test conditions were the same as [TEST 1].Test results are shown in [Table 2] below.

TABLE 2 coating layer cutting result CVD coated before electron beamelectron beam tool-life cutting insert surface treatment surfacetreatment dry/wet (minute) remark cutting TiN/MTCN/TiCN/ with dry 22 —insert A κ-Al₂O₃ without 18 — cutting TiN/MTCN/TiCN/ with wet 37 —insert B κ-Al₂O₃/TiN without 32 chipping insert: CNMG120412 workmaterial: carbon steel(HB215) cutting condition: v = 300 rpm, f = 0.25mm/rev, d = 2.0

As can be seen from [Table 2], tool lives of the cutting inserts A andB, the surfaces of which were not treated by the electron beam, arelonger than those of the cutting insert A and the cutting insert B whosesurface was treated by the electron beam.

As can be seen from [Table 1] and [Table 2], the cutting tool, which issurface treated by the electron beam in accordance with the presentinvention, has an enhanced tool life compared to that of a cutting toolwhose surface is not treated by the electron beam under the same cuttingconditions. This is a result of the coating layer having a surface,which is minute and flat, due to the electron beam surface treatment.Such a treatment reduces the cutting resistance between the workpiece aswell as the adherence of the workpiece to the cutting tool.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various alternations ormodifications can be made without departing from the scope of thepresent invention.

INDUSTRIAL APPLICABILITY

According to the surface treatment method with an electron beam of thepresent invention, when the outermost coating layer of a ceramic coatinglayer is a κ-Al₂O₃ coating layer, a certain thickness (microns) of theκ-Al₂O₃ coating layer surface will be melted, evaporated and solidifiedby the electron beam, and then be transformed into an α-Al₂O₃.Furthermore, if a laminated order of the outermost coating layers of aceramic coating layer are TiN and κ-Al₂O₃ consecutively, then the TiNlayer will be evaporated by the electron beam and at least a portion ofthe outermost surface of the κ-Al₂O₃ coating layer will be transformedinto an oα-Al₂O₃. In such a case, other coating layers below the κ-Al₂O₃coating layer and the body of the cutting tool are not influenced by theelectron beam. Thus, an α-Al₂O₃ coating layer may be formed on a surfaceof a cutting tool in an easy and stable manner.

Furthermore, according to the surface treatment method with an electronbeam of the present invention, when the outermost coating layer of aceramic coating layer is an α-Al₂O₃ coating layer, at least a portion ofit will be melted. Then, the melted surface will be solidified with itssurface flattened by the surface tension provided in the meltedcondition. Thus, the surface roughness of the coating layer can beimproved predominantly. Further, if a laminated order of the outermostcoating layer of a ceramic coating layer is TiN and α-Al₂O₃ in turn,then the TiN layer will be evaporated by the electron beam and at leasta portion of the outermost surface of the α-Al₂O₃ coating layer will bemelted, the melted surface thereof being solidified with its surfaceflattened by the surface tension provided in the melted condition. Thus,a surface roughness of the coating layer can be improved predominantly.As such, an α-Al₂O₃ coating layer is formed on a surface of a cuttingtool in an easy and stable manner. Moreover, an improved surfaceroughness reduces the cutting resistance and adherence with theworkpiece, which provides the cutting tool with a significantly improvedtool life.

1. A method of treating a surface of a cutting tool, comprising:providing one or more ceramic coating layers on the surface of thecutting tool; and irradiating an electron beam to the ceramic coatinglayer to instantaneously melt at least a portion of the outermostcoating layer of the ceramic coating layer.
 2. The method of claim 1,wherein the outermost coating layer is a κ-Al₂O₃ coating layer or a TiNcoating layer disposed on a κ-Al₂O₃ coating layer.
 3. The method ofclaim 1, wherein the outermost coating layer is an α-Al₂O₃ coating layeror a TiN coating layer disposed on an α-Al₂O₃ coating layer.
 4. Themethod of claim 2, wherein if the outermost coating layer is a κ-Al₂O₃coating layer, then at least a portion of the κ-Al₂O₃ coating layer istransformed into an α-Al₂O₃.
 5. The method of claim 2, wherein theoutermost coating layer is a TiN coating layer disposed on a κ-Al₂O₃coating layer, at least a portion of the TiN coating layer is evaporatedthrough the electron beam irradiation, and the κ-Al₂O₃ coating layertherebelow is transformed into α-Al₂O₃.
 6. The method of claim 1,wherein the ceramic coating layers are multiple layers ofTi-carbonitride formed on the substrate of the cutting tool and aκ-Al₂O₃ coating layer formed on the multiple layers of Ti-carbonitride,or are multiple layers of Ti-carbonitride formed on the substrate of thecutting tool and a κ-Al₂O₃/TiN coating layer formed on the multiplelayers of Ti-carbonitride.
 7. The method of claim 1, wherein the ceramiccoating layers are multiple layers of Ti-carbonitride formed on thesubstrate of the cutting tool and a TiCNO/α-Al₂O₃ coating layer formedon the multiple layers of Ti-carbonitride, or are multiple layers ofTi-carbonitride formed on the substrate of the cutting tool and aTiCNO/α-Al₂O₃/TiN coating layer formed on the multiple layers ofTi-carbonitride.
 8. The method of claim 1, wherein the ceramic coatinglayers are multiple layers of Ti-carbonitride formed on the substrate ofthe cutting tool and a TiCO/α-Al₂O₃ coating layer formed on the multiplelayers of Ti-carbonitride, or are multiple layers of Ti-carbonitrideformed on the substrate of the cutting tool and a TiCO/α-Al₂O₃/TiNcoating layer formed on the multiple layers of Ti-carbonitride.
 9. Themethod of claim 1, wherein a laminated order of the ceramic coatinglayer is TiN/MTCN/TiCN/κ-Al₂O₃ or TiN/MTCN/TiCN/κ-Al₂O₃/TiN from thesubstrate of the cutting tool.
 10. The method of claim 1, wherein alaminated order of the ceramic coating layer isTiN/MTCN/TiCN/TiCNO/α-Al₂O₃ or TiN/MTCN/TiCN/TiCNO/α-Al₂O₃/TiN from thesubstrate of the cutting tool.
 11. The method of claim 1, wherein alaminated order of the ceramic coating layer isTiN/MTCN/TiCN/TiCO/α-Al₂O₃ or TiN/MTCN/TiCN/TiCO/α-Al₂O₃/TiN from thesubstrate of the cutting tool.
 12. The method of claim 2, wherein theκ-Al₂O₃ coating layer has a thickness of about 2 μm to 15 μm.
 13. Themethod of claim 12, wherein at least 1 μm to 4 μm of the thickness ofthe κ-Al₂O₃ coating layer is transformed into an α-Al₂O₃ coating layerthrough the electron beam irradiation.
 14. The method of claim 1,wherein the electron beam is irradiated with a diameter of 50 mm to 100mm and a power of 25KV to 45KV.
 15. The method of claim 1, wherein theelectron beam is irradiated repeatedly 1 to 10 times depending on areaction state of the surface of the ceramic coating layer.
 16. Themethod of claim 1, wherein the electron beam is irradiated from anelectron beam generator at a vacuum atmosphere injected with a smallquantity of argon or nitrogen gas.
 17. A multilayer ceramic coatingcutting tool, comprising a κ-Al₂O₃ coating layer and an α-Al₂O₃ coatinglayer to which a portion of the κ-Al₂O₃ coating layer is transformed,wherein the α-Al₂O₃ coating layer is the outermost coating layer. 18.The multilayer ceramic coating cutting tool of claim 17, wherein theκ-Al₂O₃ coating layer has a thickness of about 2 μm to 12 μm and theα-Al₂O₃ coating layer has a thickness of about 1 μm to 4 μm.
 19. Acoated cutting tool comprising: a substrate; at least one or moreceramic coating layers formed on the substrate; a κ-Al₂O₃ coating layerformed on said one or more ceramic coating layers; and an outermostα-Al₂O₃ coating layer formed on said κ-Al₂O₃ coating layer, wherein: theoutermost α-Al₂O₃ coating layer is formed by electron beam irradiationof a portion of κ-Al₂O₃ originally belonging to said κ-Al₂O₃ coatinglayer, whereby said portion of κ-Al₂O₃ is transformed into α-Al₂O₃. 20.The coated cutting tool of claim 19, wherein: the outermost α-Al₂O₃coating layer has a surface roughness that is enhanced compared to thatof the κ-Al₂O₃ coating layer prior to electron beam irradiation.